This invention relates to an apparatus and method for providing adjustment of the operating temperatures and temperature differential of a curie effect temperature sensing switch. In the presently preferred embodiment, this invention relates to an apparatus and method for controlling the temperature of a reagent refrigerator by using a curie effect temperature sensing switch and providing external, incremental adjustment of the temperature switch's operating temperatures and temperature differential.
Some reagents and other substances that are intended for medical diagnostic uses must be stored in a narrow temperature range. For example, some diagnostic reagents must be stored in a constant 2.degree.-8.degree. C. environment. Control of the reagent temperature is critical. If the reagent becomes too warm, its life may be shortened.
Refrigerating machines have been developed to provide a controlled temperature environment for such reagents. These machines typically include a cold plate on which the reagents rest and means for controlling the cold plate temperature. Thus, the cold plate temperature controls the reagent temperature.
In one conventional reagent refrigerator, temperature control is achieved by attaching a thermistor to the cold plate. The thermistor is set in an electrical circuit so that its resistance varies as a function of the cold plate temperature. Thus, the voltage across the thermistor corresponds to the temperature of the cold plate. This voltage signal is compared with a reference voltage signal representing the desired cold plate temperature. If the compared voltages are not the same, an error correction signal is generated to adjust the cold plate temperature. The magnitude and polarity of the error correction signal are used to keep the cold plate at its desired temperature.
This thermistor/servo controller provides adjustment of the cold plate temperature range so that the refrigerator can be easily adapted for a variety of reagents with different temperature requirements. Nevertheless, this type of temperature controller also has some substantial drawbacks. First, each temperature controller usually requires its own op-amp, microprocessor interface, and supporting circuitry. The final electrical configuration is expensive and complex. Second, this type of controller provides no fail-safe protection from fatal errors. If the microprocessor fails, is halted or turned off, so that no reference voltage signal is provided, the cold plate temperature can rise and ruin the reagents.
Curie effect temperature sensing switches offer an alternative to the thermistor/servo controller. These switches typically include a reed switch, intrinsic, permanent magnets, and a ferrite material. The reed switch opens or closes in response to changes in the strength of the magnetic field in the vicinity of the reed contact gap. The ferrite material exhibits high magnetic reluctance above a first operating temperature (also called a "curie temperature") and low magnetic reluctance below a second operating temperature. One manufacturer of curie effect temperature sensing switches, Midwest Components, Inc., suggests that these switches can replace thermally actuated devices in various applications where increased life is required or protection of the switching contacts from adverse environments is deemed desirable, such as corrosive liquid temperature control.
These type of switches, however, are not readily perceived as suitable for temperature control of diagnostic reagents. These switches are made with a fixed operating temperature and temperature differential. The temperature differential is usually around 4.degree.-5.degree. C., which is too high for many temperature control applications. Moreover, these switches are not provided with means for adjusting their operating temperatures and temperature differentials. Thus, a new temperature sensor arrangement with a different operating temperature must be specially adapted for every application having a different temperature requirement.